: TRAIL (tumor necrosis factor-related apoptosis inducing ligand) is a member of the tumor necrosis factor superfamily. It can kill a variety of tumor cells but has no appreciable effect on most normal cells. Several TRAIL receptors have been identified in human. These include DR4, DR5, DcR1 and DcR2. DR4 and DR5 contain a death domain in their cytoplasmic tails and can initiate cell death when over-expressed. In contrast, DcRl and DcR2 do not encode any death domain and cannot initiate apoptosis. These so-called decoy receptors, DcR1 and DcR2, can instead act as a dominant negative mutant for DR4 and DR5, at least in transient transfection experiments. The physiological function of TRAIL and its receptors are poorly understood. Recently, soluble human DRS protein was shown to exacerbate collagen-induced autoimmunity and exogenous TRAIL was shown to inhibit T cell proliferation. Thus, TRAIL might act as a negative regulatory molecule for the immune system. To understand the physiological function of TRAIL and its receptor(s), a mouse cDNA clone that encodes a protein with equal homology to human DR4 and DR5 was isolated (rnDR4/5). An alternative splicing of the same gene was found to yield an identical protein without a death domain (mDcR). Transient transfection of mDR4I5 but not mDcR leads to apoptosis, and stable expression of mDR4/5 in FADD+ cells confers TRAIL sensitivity. To elucidate the role of this TRAIL receptor and its cytoplasmic-truncated form (mDcR) in the immune system, mice lacking both proteins and mice lacking DcR will be generated. Tissue-specific null mDR4/5 will also be made using the cre-loxP technology. The effect of DR4/5 and DcR mutation on lymphocyte development, T and B cell tolerance and immune responses will be examined. Similar to FasL, TRAIL-mediated apoptosis is mediated through FADD. This raises the question to how the same cells can be resistant to TRAIL but are sensitive to FasLmediated apoptosis. A receptor cytoplasmic-tail swapping experiment to dissect the underlying reasons for this is proposed. Retroviruses encoding mDR4/5-Fas (DR4/5 extracellular domain fused to the Fas death domain) or the reciprocal Fas-mDR4/5 chimeric proteins will be used to infect wild type or lpr/lpr T cells. The apoptotic effect of TRAIL or FasL on T cells expressing these chimeric proteins will be assessed. Successful completion of these experiments should lead to a significant understanding of how cancer differs from normal cells and how TRAIL receptor functions in the immune system.